1. Field of the Invention
The present invention relates to a deformable mirror having a deformable reflective surface, and an optical apparatus and a recording/reproducing apparatus using the same. More particularly, the present invention relates to a deformable mirror enabling accurate recording and reproducing operations with respect to an optical disk including a substrate having an arbitrary thickness and a method for fabricating the same and an optical apparatus and a recording/reproducing apparatus using the same.
2. Description of the Related Art
In recent years, optical disks have been more and more frequently applied to a wide field of applications including audio and video apparatuses and computers, because a large number of information signals can be recorded on an optical disk with a high density. FIG. 15 is a schematic diagram showing an exemplary arrangement of an optical pickup applicable to such apparatuses. In the optical pickup 100 shown in FIG. 15, a light beam 103, emitted from a semiconductor laser device 101, is collimated by a collimator lens 102. After the collimated light beam 103 is incident upon a beam splitter 104, the light beam 103 goes straight through the beam splitter 104, passes through a quarter-wave plate 105 and is incident upon a reflective mirror 106. The optical path of the light beam 103 is bent so as to be incident onto an objective lens 107. The light incident onto the objective lens 107 is converged by the objective lens 107, so that a light spot 109 is formed on an information recording surface of an optical disk 108 supported by a rotational motor 113.
Next, the light beam 110, reflected by the optical disk 108, is returned to the objective lens 107 and passes through the objective lens 107, the reflective mirror 106 and the quarter-wave plate 105 and again is incident upon the beam splitter 104. The reflected light beam 110 is reflected by the polarizing beam splitter 104 because the light beam 110 has a polarization that is different from that of the light beam 103 because the light beam 110 has passed through the quarter-wave plate 105 twice. The reflected light beam 110 from the beam splitter 104 is converged by a converging lens 111 and then received by a photodetector 112. The photodetector 112 detects the intensity of the reflected light beam 110, thereby detecting a reproduced signal.
The objective lens 107 which is usable for an optical pickup having such an arrangement is generally designed in view of the thickness of the optical disk 108. However, in the case where one attempts to apply such an objective lens 107 to an optical disk 108 having a thickness different from the designed value, a spherical aberration is caused, so that the imaging ability thereof degrades and the recording and reproducing operations cannot be performed. In various kinds of conventional optical disks including a compact disk, a video disk and other disks applicable to a magneto-optical disk apparatus or the like conformable to an International Standardization Organization (ISO) standard, the thickness thereof has been set to be substantially the same value (i.e., about 1.2 mm). Thus, it has heretofore been possible for the same optical pickup to record and reproduce information onto/from different kinds of optical disks including a compact disk, a video disk and a magneto-optical disk.
However, in recent years, various kinds of methods have been newly designed for further increasing the recording density of an optical disk. For example, (1) a method for improving an optical resolution of an objective lens by increasing the numerical aperture (NA) thereof, and (2) a method in which multiple recording layers are provided have been proposed.
If the NA of an objective lens is increased in accordance with the method (1), the diameter of a converged beam is decreased in proportion thereto, then it becomes necessary to reduce the substrate thickness of a disk in order to realize approximately the same toleration for a disk skew. For example, if the NA of an objective lens is increased from about 0.5 to about 0.6, approximately the same toleration cannot be realized for a disk skew unless the substrate thickness of the disk is reduced from about 1.2 mm to about 0.6 mm.
However, in the case where the substrate thickness of a disk is reduced to such a value, if one attempts to record and reproduce information onto/from a conventional optical disk by using an objective lens corresponding to such an optical disk having a reduced substrate thickness, then the spherical aberration is increased and the light spot on the disk is adversely enlarged, so that it becomes difficult to perform recording and reproducing operations. As a result, since it is no longer possible to maintain a compatibility between such a thin disk and a conventional optical disk, two separate optical pickups must be used for recording and reproducing information onto/from a thin optical disk and a conventional optical disk, respectively.
On the other hand, in the case of using a multi-layer disk in which a plurality of recording layers are provided via a transparent substrate having a certain thickness in accordance with the method (2), the recording capacity per disk is considerably increased. However, since an optical pickup is required to deal with different distances from the objective lens to the respective recording layers, a single optical pickup cannot correctly record and reproduce information onto/from each recording layer in such a multi-layer disk.
As a method for solving such problems, a method for correcting a substrate thickness by using a deformable mirror is known from Japanese Laid-Open Patent Publication No. 5-151591. FIG. 16 is a schematic diagram showing an optical system of a disk apparatus using such a deformable mirror.
As shown in FIG. 16, the beam 103 emitted from the semiconductor laser device 101 passes through the collimator lens 102 to reach the beam splitter 104. The beam 103 has such a polarization that the beam splitter 104 allows the beam 103 to pass through. Thus, the beam 103 passes through the beam splitter 104 and the quarter-wave plate 105 and is incident upon a beam splitter 152. By passing through the quarter-wave plate 105, the polarization of the beam 103 is changed so that the beam 103 can pass through the beam splitter 152. Therefore, the beam 103 also passes through the beam splitter 152 to reach a quarter-wave plate 151.
After passing through the quarter-wave plate 151, the beam 103 reaches a deformable mirror 150. The deformable mirror 150 is a mirror having a deformable reflective surface. When an optical disk 108 having a larger substrate thickness is installed, the surface of the mirror 150 is deformed by a deformable mirror driver circuit 153, thereby applying a spherical aberration to the beam 103, which cancels the spherical aberration caused by the increase in the substrate thickness.
The beam 103, which has been reflected by the deformable mirror 150, returns through the quarter-wave plate 151 and is reflected by the beam splitter 152 to reach the objective lens 107. The light incident upon the objective lens 107 is converged by the objective lens 107 to form the light spot 109 on the surface of an information recording medium or the optical disk 108.
Then, the light beam 110 is reflected by the optical disk 108 and again passes through the objective lens 107 and is incident upon the beam splitter 152. The light beam 110 is reflected by the beam splitter 152 to pass through the quarter-wave plate 151 so that the polarization of the light beam 110 is changed due to the function of the quarter-wave plate 151. Then, the beam 110 is reflected by the deformable mirror 150 and passes through the quarter-wave plate 151 again to be incident upon the beam splitter 152. Since the polarization of the beam 110 is changed while passing through the quarter-wave plate 151, the beam 110 is transmitted by the beam splitter 152.
Then, the light beam 110 passes through the quarter-wave plate 105 and is incident upon the beam splitter 104. The light beam 110 is reflected by the beam splitter 104, converged by the converging lens 111 and then received by the photodetector 112. The photodetector 112 detects the intensity of the reflected light beam 110, thereby detecting a reproduced signal.
FIG. 17 illustrates a specific configuration of the deformable mirror 150 described in Japanese Laid-Open Patent Publication No. 5-151591 and "Adaptive Optics for Optimization of Image Resolution" (J. P. Gaffarel et al., Applied Optics, vol. 26, pp. 3772-3777, (1987)).
The deformable mirror 150 includes: a deformable plate 181 having a mirror surface 180 thereon; a plurality of piezoelectric actuators 182 pressing against the reverse surface of the deformable plate 181 at several points; and a base substrate on which the deformable plate 181 and the piezoelectric actuators 182 are fixed. By varying a voltage to be applied to the respective piezoelectric actuators 182, the surface of the deformable plate 181 can be displaced by a desired amount, so that the mirror surface 180 of the deformable plate 181 can be deformed into a desired shape.
However, in a conventional deformable mirror using the piezoelectric actuators 182 such as that shown in FIG. 17, whenever the driving voltage is varied, the displacement of the deformable plate 181 is also varied. Particularly, when a variation is caused among the voltages of the piezoelectric actuators 182, the shape of the deformable mirror surface 180 deviates substantially from its desired shape.
In addition, the pressing force of the respective piezoelectric actuators 182 also varies because of thermal expansion caused by the variation in the ambient temperature, so that the shape of the deformable mirror surface 180 adversely deviates from its desired shape.
Furthermore, since the diameter of a light beam, which is subjected to the aberration correction, is about 4 mm, a large number of piezoelectric actuators 182 are required to be provided so as to correspond to one light beam having a diameter of about 4 mm for deforming the deformable mirror into a correct shape. As a result, the assembly process for such a deformable mirror becomes adversely complicated and the size of the entire deformable mirror or the optical pickup is disadvantageously increased because a large number of piezoelectric actuators 182 are required to be fixed and the wiring is required to be extended a longer distance.